I. Field of the Invention
The present invention relates to communication systems, particularly to a method and apparatus for performing handoff between two sectors of a common base station.
II. Description of the Related Art
In a code division multiple access (CDMA) cellular telephone, wireless local loop or personal communications system, a common frequency band is used for communication with all base stations in a system. The common frequency band allows simultaneous communication between a remote unit and more than one base station. Signals occupying the common frequency band are discriminated at the receiving station through the spread spectrum CDMA waveform properties based on the use of a high speed pseudonoise (PN) code. The high speed PN code is used to modulate signals transmitted from both the base stations and the remote units. Transmitter stations using different PN codes or PN codes that are offset in time produce signals that can be separately received at the receiving station. The high speed PN modulation also allows the receiving station to receive several instances of a common signal from a single transmitting station where the signal has traveled over several distinct propagation paths due to the multipath characteristics of the radio channel or purposefully introduced diversity.
The multipath characteristics of the radio channel create multipath signals that traverse several distinct propagation paths between the transmitting station and the receiving station. One characteristic of a multipath channel is the time spread introduced in a signal that is transmitted through the channel. For example, if an ideal impulse is transmitted over a multipath channel, the received signal appears as a stream of pulses. Another characteristic of the multipath channel is that each path through the channel may cause a different attenuation. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different signal strength than the other received pulses. Yet another characteristic of the multipath channel is that each path through the channel may cause a different phase on the signal. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different phase than the other received pulses.
In the radio channel, the multipath is created by reflection of the signal from obstacles in the environment, such as buildings, trees, cars, and people. In general the radio channel is a time varying multipath channel due to the relative motion of the structures that create the multipath. For example, if an ideal impulse is transmitted over the time varying multipath channel, the received stream of pulses would change in time location, attenuation, and phase as a function of the time that the ideal impulse is transmitted.
The multipath characteristics of a channel can cause signal fading. Fading is the result of the phasing characteristics of the multipath channel. A fade occurs when multipath vectors add destructively, yielding a received signal that is smaller than either individual vector. For example if a sine wave is transmitted through a multipath channel having two paths where the first path has an attenuation factor of X dB (decibels), a time delay of .delta. with a phase shift of .THETA. radians, and the second path has an attenuation factor of X dB, a time delay of .delta. with a phase shift of .THETA.+.pi. radians, no signal would be received at the output of the channel.
In narrow band modulation systems such as the analog FM modulation employed by conventional radio telephone systems, the existence of multiple path in the radio channel results in severe multipath fading. As noted above with a wideband CDMA, however, the different paths may be discriminated at the receiving station in the demodulation process. The discrimination of multipath signals not only greatly reduces the severity of multipath fading but provides an advantage to the CDMA system.
In an exemplary CDMA system, each base station transmits a pilot signal having a common PN spreading code that is offset in code phase from the pilot signal of other base stations. During system operation, the remote unit is provided with a list of code phase offsets corresponding to neighboring base stations surrounding the base station through which communication is established. The remote unit is equipped with a searching element that allows the remote unit to track the signal strength of the pilot signal from a group of base stations including the neighboring base stations.
A method and system for providing a communication with a remote unit through more than one base station during the handoff process are disclosed in U.S. Pat. No. 5,267,261, entitled "MOBILE ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATION SYSTEM," issued Nov. 30, 1993 assigned to the assignee of the present invention. Using this system, communication between the remote unit and the end user is uninterrupted by the eventual handoff from an original base station to a subsequent base station. This type of handoff may be considered as a "soft" handoff in that communication with the subsequent base station is established before communication with the original base station is terminated. When the remote unit is in communication with two base stations, the remote unit combines the signals received from each base station in the same manner that multipath signals from a common base station are combined.
In a typical macrocellular system, a system controller may be employed to create a single signal for the other end user from the signals received by each base station. Within each base station, signals received from a common remote unit may be combined before they are decoded and thus take full advantage of the multiple signals received. The decoded result from each base station is provided to the system controller. Once a signal has been decoded it cannot be `combined` with other signals. Thus the system controller must select between the plurality of decoded signals produced by each base station with which communication is established by a single remote unit. The most advantageous decoded signal is selected from the set of signals from the base stations and the other signals are simply discarded.
Remote unit assisted soft handoff operates based on the pilot signal strength of several sets of base stations as measured by the remote unit. The Active Set is the set of base stations through which active communication is established. The Neighbor Set is a set of base stations surrounding an active base station comprising base stations that have a high probability of having a signal strength of sufficient level to establish communication. The Candidate Set is a set of base stations having a pilot signal strength at a sufficient signal level to establish communication.
When communications are initially established, a remote unit communicates through a first base station and the Active Set contains only the first base station. The remote unit monitors the pilot signal strength of the base stations of the Active Set, the Candidate Set, and the Neighbor Set. When a pilot signal of a base station in the Neighbor Set exceeds a predetermined threshold level, the base station is added to the Candidate Set and removed from the Neighbor Set at the remote unit. The remote unit communicates a message to the first base station identifying the new base station. A cellular or personal communication system controller decides whether to establish communication between the new base station and the remote unit. Should the cellular or personal communication system controller decide to do so, the cellular or personal communication system controller sends a message to the new base station with identifying information about the remote unit and a command to establish communications therewith. A message is also transmitted to the remote unit through the first base station. The message identifies a new Active Set that includes the first and the new base stations. The remote unit searches for the new base station transmitted information signal and communication is established with the new base station without termination of communication through the first base station. This process can continue with additional base stations.
When the remote unit is communicating through multiple base stations, it continues to monitor the signal strength of the base stations of the Active Set, the Candidate Set, and the Neighbor Set. Should the signal strength corresponding to a base station of the Active Set drop below a predetermined threshold for a predetermined period of time, the remote unit generates and transmits a message to report the event. The cellular or personal communication system controller receives this message through at least one of the base stations with which the remote unit is communicating. The cellular or personal communication system controller may decide to terminate communications through the base station having a weak pilot signal strength.
The cellular or personal communication system controller upon deciding to terminate communications through a base station generates a message identifying a new Active Set of base stations. The new Active Set does not contain the base station through which communication is to be terminated. The base stations through which communication is established send a message to the remote unit. The cellular or personal communication system controller also communicates information to the base station to terminate communications with the remote unit. The remote unit communications are thus routed only through base stations identified in the new Active Set.
Because the remote unit is communicating with the end user though at least one base station at all times throughout the soft handoff process, no interruption in communication occurs between the remote unit and the end user. A soft handoff provides significant benefits in its inherent "make before break" technique over the conventional "break before make" technique employed in other cellular communication systems.
In a cellular or personal communication telephone system, maximizing the capacity of the system in terms of the number of simultaneous telephone calls that can be handled is extremely important. System capacity in a spread spectrum system can be maximized if the transmission power of each remote unit is controlled such that each transmitted signal arrives at the base station receiver at the same level. In an actual system, each remote unit may transmit the minimum signal level that produces a signal-to-noise ratio that allows acceptable data recovery. If a signal transmitted by a remote unit arrives at the base station receiver at a power level that is too low, the bit-error-rate may be too high to permit high quality communications due to interference from the other remote units. On the other hand, if the remote unit transmitted signal is at a power level that is too high when received at the base station, communication with this particular remote unit is acceptable but this high power signal acts as interference to other remote units. This interference may adversely affect communications with other remote units.
Therefore to maximize capacity in an exemplary CDMA spread spectrum system, the transmit power of each remote unit within the coverage area of a base station is controlled by the base station to produce the same nominal received signal power at the base station. In the ideal case, the total signal power received at the base station is equal to the nominal power received from each remote unit multiplied by the number of remote units transmitting within the coverage area of the base station plus the power received at the base station from remote units in the coverage area of neighboring base stations.
The path loss in the radio channel can be characterized by two separate phenomena: average path loss and fading. The forward link, from the base station to the remote unit, operates on a different frequency than the reverse link, from the remote unit to the base station. However because the forward link and reverse link frequencies are within the same general frequency band, a significant correlation between the average path loss of the two links exists. On the other hand, fading is an independent phenomenon for the forward link and reverse link and varies as a function of time.
In an exemplary CDMA system, each remote unit estimates the path loss of the forward link based on the total power at the input to the remote unit. The total power is the sum of the power from all base stations operating on the same frequency assignment as perceived by the remote unit. From the estimate of the average forward link path loss, the remote unit sets the transmit level of the reverse link signal. Should the reverse link channel for one remote unit suddenly improve compared to the forward link channel for the same remote unit due to independent fading of the two channels, the signal as received at the base station from this remote unit would increase in power. This increase in power causes additional interference to all signals sharing the same frequency assignment. Thus a rapid response of the remote unit transmit power to the sudden improvement in the channel would improve system performance. Therefore it is necessary to have the base station continually contribute to the power control mechanism of the remote unit.
Remote unit transmit power may also be controlled by one or more base stations. Each base station with which the remote unit is in communication measures the received signal strength from the remote unit. The measured signal strength is compared to a desired signal strength level for that particular remote unit. A power adjustment command is generated by each base station and sent to the remote unit on the forward link. In response to the base station power adjustment command, the remote unit increases or decreases the remote unit transmit power by a predetermined amount. By this method, a rapid response to a change in the channel is effected and the average system performance is improved. Note that in a typical cellular system, the base stations are not intimately connected and each base station in the system is unaware of the power level at which the other base stations receive the remote unit's signal.
When a remote unit is in communication with more than one base station, power adjustment commands are provided from each base station. The remote unit acts upon these multiple base station power adjustment commands to avoid transmit power levels that may adversely interfere with other remote unit communications and yet provide sufficient power to support communication from the remote unit to at least one of the base stations. This power control mechanism is accomplished by having the remote unit increase its transmit signal level only if every base station with which the remote unit is in communication requests an increase in power level. The remote unit decreases its transmit signal level if any base station with which the remote unit is in communication requests that the power be decreased. A system for base station and remote unit power control is disclosed in U.S. Pat. No. 5,056,109 entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM," issued Oct. 8, 1991, assigned to the Assignee of the present invention.
Base station diversity at the remote unit is an important consideration in the soft handoff process. The power control method described above operates optimally when the remote unit communicates with each base station through which communication is possible. In doing so, the remote unit avoids inadvertently interfering with communications through a base station receiving the remote unit's signal at an excessive level but unable to communicate a power adjustment command to the remote unit because communication is not established therewith.
A typical cellular or personal communication system contains some base stations having multiple sectors. A multi-sectored base station comprises multiple independent transmit and receive antennas. The process of simultaneous communication with two sectors of the same base station is called softer handoff. The process of soft handoff and the process of softer handoff are the same from the remote unit's perspective. However the base station operation in softer handoff is different from soft handoff. When a remote unit is communicating with two sectors of the same base station, the demodulated data signals of both sectors are available for combination within the base station before the signals are passed to the cellular or personal communication system controller. Because the two sectors of a common base station share circuitry and controlling functions, a variety of information is readily available to sectors of a common base station that is not available between independent base stations. Also two sectors of a common base station send the same power control information to a remote unit (as discussed below).
The combination process in softer handoff allows demodulated data from different sectors to be combined before decoding and thus produce a single soft decision output value. The combination process can be performed based on the relative signal level of each signal thus providing the most reliable combination process.
As noted above, the base station can receive multiple instances of the same remote unit signal. Each demodulated instance of the arriving signal is assigned to a demodulation element. The demodulated output of the demodulation element is combined. The combined signal is decoded. The demodulation elements, instead of being assigned to a single sector, may be assigned to a signal from any one of a set of sectors in the base station. Thus, the base station may use it resources with high efficiency by assigning demodulation elements to the strongest signals available.
Combining signals from sectors of a common base station also allows a sectorized base station to make a single power adjustment command for remote unit power control. Thus the power adjustment command from each sector of a common base station is the same. This uniformity in power control allows flexible handoff operation in that sector diversity at the remote unit is not critical to the power control process. Further details of the softer handoff process are disclosed in U.S. Patent entitled "METHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION," issued Apr. 29, 1997, assigned to the assignee of the present invention. Further information on the benefits and application of softer handoff are disclosed in U.S. patent application Ser. No. 08/144,901, filed Oct. 30, 1993, entitled "METHOD AND APPARATUS FOR REDUCING THE AVERAGE TRANSMIT POWER FROM A SECTORIZED BASE STATION" and U.S. patent application Ser. No. 08/316,155, filed Sep. 30, 1994 entitled "METHOD AND APPARATUS FOR REDUCING THE AVERAGE TRANSMIT POWER OF A BASE STATION" each of which is assigned to the assignee of the present invention.
Each base station in the cellular system has a forward link coverage area and a reverse link coverage area. These coverage areas define the physical boundary beyond which base station communication with a remote unit is degraded. In other words, if a remote unit is within the base station's coverage area, the remote unit can communicate with the base station, but if the remote unit is beyond the coverage area, communications are compromised. A base station may have single or multiple sectors. Single sectored base stations have approximately a circular coverage area. Multi-sectored base stations have independent coverage areas that form lobes radiating from the base station.
Base station coverage areas have two handoff boundaries. A handoff boundary is defined as the physical location between two base stations where the link would perform the same regardless of whether the remote unit is communicating with the first or second base station. Each base station has a forward link handoff boundary and a reverse link handoff boundary. The forward link handoff boundary is defined as the location where the remote unit's receiver would perform the same regardless of which base station it was receiving. The reverse link handoff boundary is defined as the location of the remote unit where two base station receivers would perform the same with respect to that remote unit.
Ideally these boundaries should be balanced, meaning that they should have the same physical location. If they are not balanced, system capacity may be reduced as the power control process is disturbed or the handoff region unreasonably expands. Note that handoff boundary balance is a function of time, in that the reverse link coverage area shrinks as the number of remote units present therein increases. Reverse link power, which increases with each additional remote unit, is inversely proportional to the reverse link coverage area. An increase in receive power decreases the effective size of the reverse link coverage area of the base station and causes the reverse link handoff boundary to move inward toward the base station.
To obtain high performance in a CDMA or other cellular system, it is important to carefully and accurately control the transmit power level of the base stations and remote units in the system. Transmit power control limits the amount of self-interference produced by the system. Moreover, on the forward link, a precise level of transmit power can serve to balance the forward and reverse link handoff boundaries of a base station or a single sector of a multi-sectored base station. Such balancing helps to reduce the size of the handoff regions, increase overall system capacity, and improve remote unit performance in the handoff region.
Before adding a new base station to the existing network, the forward link (i.e., transmit) power and the reverse link (i.e., receive) signal power of the new base station are both approximately equal to zero. To begin the process of adding the new base station, an attenuator in the receive path of the new base station is set to a high attenuation level, creating a high level of artificial noise receive power. An attenuator in the transmit path is also set to a high attenuation level, which in turn causes a low transmit power level. The high level of artificial noise receive power results in the reverse link coverage area of the new base station being very small. Similarly, because the forward link coverage area is directly proportional to the transmit power, the very low transmit power level and the forward link coverage area is also very small.
The process then continues by adjusting the attenuators in the receive and transmit paths in unison. The attenuation level of the attenuator in the receive path is decreased, thereby decreasing the level of artificial noise receive power, increasing the natural signal level, and hence increasing the size of the reverse link coverage area. The attenuation level of the transmit path attenuator is also decreased, thereby increasing the transmit power level of the new base station and expanding its forward link coverage area. The rate at which the transmit power is increased and the artificial noise receive power is decreased must be sufficiently slow to permit handoff of calls between the new and surrounding base stations as the new base station is added to or removed from the system.
Each base station in the system is initially calibrated such that the sum of the unloaded receiver path noise measured in decibels and the desired pilot power measured in decibels is equal to some constant. The calibration constant is consistent throughout the system of base stations. As the system becomes loaded (i.e., remote units begin to communicate with the base stations), a compensation network maintains the constant relationship between the reverse link power received at the base station and the pilot power transmitted from the base station. The loading of a base station effectively moves the reverse link handoff boundary closer in toward the base station. Therefore to imitate the same effect on the forward link, the pilot power is decreased as loading is increased. The process of balancing the forward link handoff boundary to the reverse link handoff boundary is referred to as base station breathing is detailed in U.S. Pat. No. 5,548,812 entitled "METHOD AND APPARATUS FOR BALANCING THE FORWARD LINK HANDOFF BOUNDARY TO THE REVERSE LINK HANDOFF BOUNDARY IN A CELLULAR COMMUNICATION SYSTEM" issued Aug. 20, 1996 and assigned to the assignee of the present invention. The process of balancing the forward link handoff boundary to the reverse link handoff boundary during the addition or removal of a base station from a system is referred to as base station blossoming and wilting is detailed in U.S. Pat. No. 5,475,870 entitled "APPARATUS AND METHOD FOR ADDING AND REMOVING A BASE STATION FROM A CELLULAR COMMUNICATION SYSTEM" issued Dec. 12, 1995 and assigned to the assignee of the present invention.
It is desirable to control the relative power used in each forward link signal transmitted by the base station in response to control information transmitted by each remote unit. The primary reason for providing such control is to accommodate the fact that in certain locations the forward link may be unusually disadvantaged. Unless the power being transmitted to the disadvantaged remote unit is increased, the signal quality may become unacceptable. An example of such a location is a point where the path loss to one or two neighboring base stations is nearly the same as the path loss to the base station communicating with the remote unit. In such a location, the total interference would be increased by three times over the interference seen by a remote unit at a point relatively close to its base station. In addition, the interference coming from the neighboring base stations does not fade in unison with the signal from the active base station as would be the case for interference coming from the active base station. A remote unit in such a situation may require 3 to 4 dB of additional signal power from the active base station to achieve adequate performance.
At other times, the remote unit may be located where the signal-to-interference ratio is unusually good. In such a case, the base station could transmit the corresponding forward link signal using a lower than nominal transmitter power, reducing interference to other signals being transmitted by the system.
To achieve the above objectives, a signal-to-interference measurement capability can be provided within the remote unit receiver. A signal-to-interference measurement is performed by comparing the power of the desired signal to the total interference and noise power. If the measured ratio is less than a predetermined value, the remote unit transmits a request to the base station for additional power on the forward link. If the ratio exceeds the predetermined value, the remote unit transmits a request for power reduction. One method by which the remote unit receiver can monitor signal-to-interference ratios is by monitoring the frame error rate (FER) of the resulting signal.
The base station receives the power adjustment requests from each remote unit and responds by adjusting the power allocated to the corresponding forward link signal by a predetermined amount. The adjustment would usually be small, typically on the order of 0.5 to 1.0 dB, or around 12%. The rate of change of power may be somewhat slower than that used for the reverse link, perhaps once per second. In the preferred embodiment, the dynamic range of the forward link adjustment is typically limited such as from 4 dB less than nominal to about 6 dB greater than nominal transmit power.
CDMA base stations have the ability to provide accurate control over the power level at which they transmit. To provide accurate power control, it is necessary to compensate for variations in the gain in the various components comprising the transmit chain of the base station. Variations in the gain typically occur over temperature and aging such that a simple calibration procedure at deployment does not guarantee a precise level of output transmit power over time. Variations in the gain can be compensated by adjusting the overall gain in the transmit chain so that the actual transmit power of the base station matches a calculated desired transmit power. Each base station sector produces several signaling channels that operate at a variety of data rates and relative signal levels that combined create a raw radio frequency transmit signal. The channel element modulators, each of which corresponds to a channel, calculate the expected power of each channel signal. The base station also comprises a base station transceiver system controller (BTSC) which generates a desired output power of the sector by summing the expected powers of each channel.
As noted above, a typical cellular system is comprised of a plurality of spaced apart base stations each having a set of associated collocated antennas. A typical cellular base station may be comprised of three or more sectors. The sectors are subdivisions of the base station that are intimately related. Each sector transmits a different set of signals than the set of signals transmitted by every other sector in the base station. Because the sector circuitry is collocated, it may be easily shared and interconnected between the sectors. The antenna pattern of a typical three sectored base station is shown in FIG. 1. In FIG. 1 coverage area 300A is represented by the finest width line. Coverage area 300B is represented by the medium width line. Coverage area 300C is represented by the heaviest line. The shape of the three coverage areas shown in FIG. 1 is the shape produced by standard directional dipole antennas. The edges of the coverage areas can be thought of as the location at which a remote unit receives the minimum signal level necessary to support communication through that sector. As a remote unit moves into the sector, the signal strength received from the base station as perceived by the remote unit increases. A remote unit at point 302 may communicate through sector 300A. A remote unit at point 303 may communicate through sector 300A and sector 300B. A remote unit at point 304 communicates through sector 300B. As a remote unit moves past the edge of the sector, communication through that sector may degrade. A remote unit operating in soft handoff mode between the base station in FIG. 1 and an unshown neighboring base station is likely to be located near the edge of one of the sectors.
FIG. 2 illustrates an exemplary embodiment of a standard cellular system showing three single sectored base stations 362, 364, and 368. In FIG. 2, each of antennas 310, 326, and 344 is the receive antenna for base station 362, 364, or 368 respectively. Base stations 362, 364, and 368 are in proximity to one another and antennas 310, 326, and 344 have overlapping coverage areas such that a single remote unit signal may be in soft handoff with all three base stations at one time.
Antennas 310, 326, and 344 supply a receive signal to receive processings 312, 328, and 346 respectively. Receive processings 312, 328, and 346 process the RF signal and convert the signal to digital bits. Receive processings 312, 328, and 346 may also filter the digital bits. Receive processing 312 provides the filtered digital bits to demodulation elements 316A-316N. Receive processing 328 provides the filtered digital bits to demodulation elements 332A-332N. Likewise, receive processing 346 provides the filtered digital bits to demodulation elements 350A-350N.
Demodulation elements 316A-316N are controlled by controller 318 through interconnection 320. Controller 318 assigns demodulation elements 316A-316N to one of the instances of information signal from the same remote unit as perceived by base station 362. The distinct instances of the signal may be created due to the multipath characteristics of the environment. Demodulation elements 316A-316N produce data bits 322A-322N that are combined in symbol combiner 324. The output of symbol combiner 324 may be aggregate soft decision data suitable for Viterbi decoding. The combined data is decoded by decoder 314 and output as Message 1 and passed to cellular or personal communications system controller 370.
A power adjustment command from base station 362 for the remote unit is created by controller 318 from the combined signal strength of all the signals demodulated by demodulation elements 316A-316N. Controller 318 can pass the power control information to the transmit circuitry (not shown) of base station 362 to be relayed to the remote unit.
Demodulation elements 332A-332N are controlled by controller 334 through interconnection 336. Controller 334 assigns demodulation elements 332A-332N to one of the instances of information signals from the same remote unit. Demodulation elements 332A-332N produce data bits 338A-338N that are combined in symbol combiner 340. The output of symbol combiner 340 may be aggregate soft decision data suitable for Viterbi decoding. The combined data is decoded by decoder 342 and output as Message 2 and passed to cellular or personal communications system controller 370.
A power adjustment command for the remote unit is created by controller 334 from the combined signal strength of all the signals demodulated by demodulation elements 332A-332N. Controller 334 can pass the power control information to the transmit circuitry (not shown) of base station 364 to be relayed to the remote unit.
Demodulation elements 350A-350N are controlled by controller 352 through interconnection 354. Controller 352 assigns demodulation elements 350A-350N to one of the instances of information signals from the same remote unit as perceived by base station 368. Demodulation elements 350A-350N produce data bits 356A-356N that are combined in symbol combiner 358. The output of symbol combiner may be aggregate soft decision data suitable for Viterbi decoding. The combined data is decoded by decoder 360 and the output as Message 3 and passed to cellular or personal communications system controller 370.
A power adjustment command for the remote unit is created by controller 352 from the estimated signal strengths of all the signals demodulated by demodulation elements 350A-350N. Controller 352 can pass the power control information to the transmit circuitry (not shown) of base station 368 to be relayed to the remote unit.
For each remote unit operating in soft handoff in the system, cellular or personal communication system controller 370 receives decoded data from at least two base stations. For example, in FIG. 2 cellular or personal communications system controller 370 receives decoded data in the form of Messages 1, 2, and 3 from the common remote unit from base stations 362, 364, and 368 respectively. The decoded data cannot be combined to yield the great advantage that is achieved by combining the data prior to decoding. Therefore typically cellular or personal communication system controller 370 does not combine the decoded data from each base station and instead selects one of the three decoded data Messages 1, 2, or 3 having the highest signal quality index and discards the other two. In FIG. 2 selector 372 performs the selection process on a frame by frame basis and provides the result to a vocoder or other data processing unit. More information about the selection process can be found in co-pending U.S. patent application Ser. No. 08/519,670 entitled "COMMUNICATION SYSTEM USING REPEATED DATA SELECTION" filed Aug. 25, 1995 and assigned to the assignee of the present invention.
The reason the combined but undecoded data output from symbol combiners 324, 340, and 358 is not sent respectively from base stations 362, 364, and 368 to system controller 370 is that the demodulation process produces data at a very high rate. A large block of data is used in the decoding process to produce the decoded symbol. The ratio of the amount of data necessary to decode a data symbol and the amount of data to specify a decoded symbol and quality index can be as high as 1000 to 1. In addition to the complexity, the inherent delay of transporting such large amounts of data is prohibitive unless a very high speed link is used. Thus the interconnection system between the hundreds of base stations in the system (most of which are not shown in FIG. 2) and system controller 370 is greatly simplified by sending only the decoded data and quality indications instead of the undecoded data suitable for combination.
Besides the complexity of transmitting the large amount of data associated with combined but undecoded data, the cost is also prohibitive. Typically the base stations of a system are remotely located from the system controller. The path from the base stations to the system control typically comprises a leased line such as a T1 interface line. The cost of these lines is largely determined by the amount of data that they carry. Thus increasing the amount of data that is transmitted from the base stations to the system controller can be cost prohibitive as well as technically difficult.
In a less than optimal system the selection method of soft handoff described with respect to FIG. 2 could be directly applied to a sectorized base station by treating each sector of a common base station as a separate, independent base station. Each sector of the base station could combine and decode multipath signals from a common remote unit. The decoded data could be sent directly to the cellular or personal communication system controller by each sector of the base station or it could be compared and selected at the base station and the result sent to the cellular or personal communication system controller. But a much more advantageous method of handling handoff between sectors of a common base station is to use softer handoff as described in the above mentioned U.S. Pat. No. 5,625,876. Circuitry for providing softer handoff is described in conjunction with FIG. 3.
In FIG. 3, each of antennas 222A-222C is the receive antenna for one sector and each of antennas 230A-230C is the transmit antenna for one sector. Antenna 222A and antenna 230A correspond to a common coverage area and can ideally have the same antenna pattern. Likewise antennas 222B and 230B, and antennas 222C and 230C correspond to common coverage areas respectfully. FIG. 3 represents a typical base station in that antennas 222A-222C have overlapping coverage areas such that a single remote unit signal may be present at more than one antenna at a time. Antennas 222A-222C may provide antenna patterns as shown in FIG. 1 or one or more of antennas 222A-222C may be distributed antennas.
Referring again to FIG. 3, antennas 222A, 222B, and 222C supply the received signal to receive processings 224A, 224B, and 224C respectively. Receive processings 224A, 224B, and 224C process the RF signal and convert the signal to digital bits. Receive processings 224A, 224B, and 224C may filter the digital bits and provide the resulting digital bits to interface port 226. Interface port 226 may connect any of the three incoming signal paths to any of the demodulation elements 204A-204N under the control of controller 200 through interconnection 212.
Demodulation elements 204A-204N are controlled by controller 200 through interconnection 212. Controller 200 assigns demodulation elements 204A-204N to one of the instances of information signals from a single remote unit from any one of the sectors. Demodulation elements 204A-204N produce data bits 220A-220N each representing an estimate of the data from the single remote unit. Data bits 220A-220N are combined in symbol combiner 208 to produce a single estimate of the data from the remote unit. The output of symbol combiner 208 may be aggregate soft decision data suitable for Viterbi decoding. The combined symbols are passed to decoder 228.
Demodulation elements 204A-204N also provide several output control signals to controller 200 through interconnection 212. The information passed to controller 200 includes an estimate of the signal strength of the signal assigned to a particular demodulation element. Each one of demodulation elements 204A-204N measures a signal strength estimation of the signal that it is demodulating and provides the estimation to controller 200.
Notice that symbol combiner 208 can combine signals from just one sector to produce an output or it can combine symbols from multiple sectors as selected by the interface port 226. A single power control command is created by controller 200 from the estimated signal strengths from all the sectors through which the signal is received. Controller 200 can pass the power control information to the transmit circuitry of each sector of the base station. Thus each sector in the base station transmits the same power control information to a single remote unit.
When symbol combiner 208 is combining signals from a remote unit that is communicating through more than one sector, the remote unit is in softer handoff. The base station may send the output of decoder 228 to a cellular or personal communication system controller. At the cellular or personal communication system controller, signals corresponding to the remote unit from this base station and from other base stations may be used to produce a single output using the selection process described above.
The transmit processing shown in FIG. 3 receives a message for a remote unit from the end user through the cellular or personal communication system controller. The message can be sent on one or more of antennas 230A-230C. Interface port 236 connects the message for the remote unit to one or more of modulation elements 234A-234C as set by controller 200. Modulation elements 234A-234C modulate the message for the remote unit with the appropriate PN code. The modulated data from modulation elements 234A-234C is passed to transmit processing 232A-232C respectively. Transmit processings 232A-232C convert the message to an RF frequency and transmit the signal at an appropriate signal level through antennas 230A-230C respectively. Notice that interface port 236 and interface port 226 operate independently in that receiving a signal from a particular remote unit through one of antennas 222A-222C does not necessarily mean that the corresponding transmit antenna 230A-230C is transmitting a signal to the particular remote unit. Also note that the power control command sent through each antenna is the same, thus sector diversity from a common base station is not critical for the optimal power control performance. These advantages are further exploited to the advantage of the system in the above mentioned U.S. patent application Ser. Nos. 08/144,901 and 08/316,155 through a process referred to as transmit gating.
Besides the complexities of power control noted above, the process of power control becomes more complicated when soft handoff between two or more base station is attempted when the two base stations are controlled by different switches. The process of breathing also complicates traditional power control mechanism. The present invention is a method and apparatus for providing power control administration through a set of base stations that are breathing and that may be controlled by a different switch.